Scientific Forecaster Discussion
fxus66 kmfr 280300
Area forecast discussion
National Weather Service Medford or
800 PM PDT Sat may 27 2017
Discussion...28/00z NAM in.
Mostly clear skies prevail across the Medford County Warning Area with the exception
of the coast and the adjacent coastal waters...where low marine
stratus dominates. This stratus will progress inland tonight into
Sunday morning...covering part of the Umpqua basin in the late
night to mid-morning hours.
The clear skies over most of the area come courtesy of a strong
long wave ridge that is now moving onshore. This is bringing dry
northerly flow aloft to the area. This ridge will move slowly
east...breaking to the east of the area Sunday afternoon into
Dry and warm weather will prevail into Sunday afternoon. Sunday
highs will be around 5 degrees below normal along the coast due to
persistent marine layer low clouds...around 10 degrees above
normal in the Umpqua basin due to morning marine layer clouds...
and near 15 degrees above normal over the rest of the area.
As the long wave ridge axis breaks to the east of the area, the
flow aloft will become southerly and this will open the door to
monsoonal moisture advecting into the area. Thus, thunderstorms
become a possibility as early as Sunday afternoon. However, the
moisture will still be very limited at that time (pws around 0.5
inches over the unstable areas), so the chances of thunderstorm
development is quite low.
Southerly flow aloft will gradually increase into Monday, and
conditions will become much more favorable for thunderstorm
development. Difluence aloft, increasing 700 mb south to
southeast flow (10-15 kts) due to an approaching slightly
negatively tilted upper level trough, increasing moisture (pws
increasing to 0.75 to around 1.10 inches), and increasing
instability will combine to support showers and thunderstorms
across much of the area. The models continue to indicate a dry
slot near Mt Shasta city Monday afternoon but otherwise
thunderstorms are possible across Siskiyou County into western
Modoc County, and from the Illinois Valley east and northeastward
across southwest and south central Oregon. One or two of these
storms could become severe.
As far as temperatures go...Monday highs will be a few degrees
cooler over the west side...and about the same as the Sunday
highs over the east side and northern California.
A short wave ejected from an offshore long wave trough and an
associated cold front will move through the area Tuesday. This
will bring in much cooler air and a slight chance to chance of
showers and thunderstorms. Unlike Monday, a majority of the
precipitation will be in the form of showers rather than
thunderstorms. The highest instability is likely to be in northern
Klamath and Northwest Lake counties northward toward Bend.
Tuesday highs will be similar to the Monday highs along the
coast...but around 10 degrees cooler over the inland west side and
around 5 degrees cooler east of the Cascades.
Long term discussion from the Saturday afternoon afd...Wednesday
through Saturday. The GFS depiction of a closed low forming
offshore of the southern Oregon coast and moving south into
Southern California yesterday now has an open upper level trough
that is very close to what the European model (ecmwf) showed yesterday, and today.
The GFS is deeper than the ec but expect that it will come more in
line with subsequent runs. This open upper level trough solution
brings winds to the west more quickly and pushes the unstable air
mass to the east side on Wednesday afternoon. Southerly flow
returns quickly Wednesday evening returning a feed of moist
unstable air and bringing a slight chance of thunderstorms to the
Cascade crest/Mt. Shasta region eastward Thursday afternoon into
the evening. With the weak upper level trough pattern remaining in
place another weak front brings showers into the region Friday,
but in a northwesterly flow that is more stable and no
thunderstorms are expected. The upper trough shifts eastward
Saturday, with drying and warming as offshore flow develops and a
thermal trough returns at the coast. -Sven
Aviation...28/00z taf cycle...along the coast and over the coastal
waters...MVFR cigs with areas of IFR cigs/vsbys will prevail over
the next 24 hours. Patchy drizzle is possible as well. VFR will
prevail elsewhere, except MVFR/IFR ceilings/fog will spread into the
coastal valleys, with MVFR ceilings expected after 09z in krbg.
The low stratus should erode back toward the immediate coast after
18z Sunday. -Spilde
Marine...updated 800 PM Saturday 27 may 2017...the marine layer
will remain quite deep over the coastal waters the rest of this
weekend resulting in widespread stratus and fog that could reduce
visibility to less than a half of a mile at times. Drizzle is also
possible. Winds and seas, however, will remain light.
A thermal trough will redevelop Sunday and north winds will
increase. They will peak (likely below Small Craft Advisory levels)
Models have really backed off on the strength of the thermal
trough idea during the middle part of the coming week. They are
now showing a disturbance moving onshore Tuesday...which would
allow the thermal trough to weaken again. Additional weak
disturbances will keep generally light, variable winds and low
seas over the area Wednesday/Thursday and perhaps even into
Friday. At this point it looks like the thermal trough will
eventually return late in the week or next weekend.
Pacific coastal waters...none.